Set Up CalDAV Connection in iPhone/iPad Calendar

To add the mail.de calendar in Apple's iCal on iPhone or iPad, follow these steps:

1. Go to "Settings / Mail", "Contacts", "Calendars / Add Account".
2. In the list, select "Other".
3. Under Calendars, tap "Add CalDAV Account".
4. Enter the following information:
   Server: https://kalender.mail.de/principals/mustermann@mail.de
   Username: your mail.de username/login
   Password: your mail.de password
   Description: Any name you like (e.g., mail.de Calendar)
5. Tap "Next".
6. The connection will be verified and your calendar will appear under "Accounts".
7. Open the Calendar app and select which calendars you want to display. Choose your mail.de calendar and tap "Done".
8. All events from this calendar will now be displayed.